This invention relates to spin welding methods and apparatus for bonding thermoplastic elements together.
Spin welding is a technique which has heretofore been employed to bond together thermoplastic elements. Various equipment and operations for spin welding have been heretofore proposed, as exemplified by the following U.S. Pat. Nos.: 2,933,428 issued to Mueller on Apr. 19, 1960; 3,078,912 issued to Hitzelberger on Feb. 26, 1963; 3,344,010 issued to Franz on Sept. 26, 1967; and 3,993,519 issued to Birkhold on Nov. 23, 1976.
Spin welding is effected by establishing a relative rubbing action between the plastic workpieces to generate frictional heat which melts the interface between the workpieces. When this melt subsequently hardens, the workpieces adhere together at the interface.
Relative rubbing movement can be effected by placing the workpieces into separate fixtures, rotating one of the fixtures, and bringing the workpieces into contact. When sufficient melting has occurred, the driven workpiece is braked. This technique is not highly efficient, due to the time and energy expended in repeatedly starting and stopping the driving fixture.
Alternatively, a continuously rotating drive fixture can be employed which is brought into axial contact with a driven workpiece that is mounted on a stationary workpiece. Consequently, full acceleration of the driven workpiece occurs rapidly. When sufficient melt has occurred, the driven workpiece is released from the drive fixture and comes to a stop upon adhering to the stationary workpiece. Thus, there is no need for repeatedly starting and stopping the drive fixture.
The latter technique does, however, present a problem involving the manner of establishing a driving relationship between the drive fixture and the driven workpiece. It has been suggested, for instance, that teeth or fingers on the drive fixture axially contact the driven workpiece or enter openings in the driven workpiece. One disadvantage of this technique is that precise control over the spin welding operation is difficult to attain. That is, the driven workpiece will tend to rotate when first contacted by the drive fixture. However, as the drive fixture presses the driven workpiece axially against the stationary workpiece, frictional resistance to rotation will intensify. Consequently, there is a tendency for some slippage and uneven acceleration of the driven workpiece to occur during start-up. These same effects function in reverse to cause slippage during release of the driven workpiece. Consequently, the time required to fully accelerate and decellerate the driven workpiece, and hence the time for effecting a weld, can vary from one operation to the next. This variance can be significant, since high-speed spin welding of plastic workpieces can be achieved in a spin period of less than a second, and since inexact periods of rotation tend to create inadequate bonds. That is, insufficient rotation will produce too little melting at the interface, while too much rotation produces excessive melt which tends to flow from the interface.
Other problems associated with the reliance upon axial drive contact between the drive fixture and driven workpiece include the danger that during slippage in the drive contact, the driven workpiece can be damaged by teeth on the drive fixture, or that undesired melting can occur where the drive fixture contacts the driven workpiece.
Therefore, it is an object of the present invention to eliminate or minimize problems of the sort discussed above.
It is another object of the invention to provide novel spin welding methods and apparatus.
It is a further object of the invention to provide novel spin welding methods and apparatus in which the spin-period is precisely controlled.
It is yet another object of the invention to provide novel spin welding methods and apparatus which effectively utilize a continuously rotating drive member.
It is a further object of the invention to provide novel spin welding methods and apparatus which minimize the risk of damage or inadvertent melting of the workpiece.
It is an additional object of the invention to provide novel spin welding methods and apparatus for bonding a thermoplastic tube to a thermoplastic fitting so that a longitudinal and radial interface therebetween is bonded together.
It is still a further object of the invention to provide novel spin welding methods and apparatus in which a driven workpiece is radially gripped by a continuously rotating collet.